<!DOCTYPE html>
<html lang="en">
  <head>
    <meta charset="UTF-8" />
    <meta http-equiv="X-UA-Compatible" content="IE=edge" />
    <meta name="viewport" content="width=device-width, initial-scale=1.0" />
    <title>Document</title>
    <style>
      article {
        height: 3000px;
        background: #fff;
      }
      div {
        height: 30px;
        position: fixed;
        /* 已知 上 左 右  和高度 则能唯一确定一个div的位置和大小*/
        top: 0;
        left: 0;
        right: 0;
        border-bottom: solid 3px blueviolet;
        box-shadow: 0 4px 5px #ddd;
      }
    </style>
  </head>
  <body>
    <article></article>
    <div></div>
  </body>
</html>
